The present invention relates generally to degradable particulates. More particularly, the present invention relates to methods for producing degradable particulates and slurries thereof, and methods related to the use of such degradable particulates and slurries in subterranean applications.
Degradable particulates comprise degradable materials (which oftentimes comprise degradable polymers) that are capable of undergoing an irreversible degradation when used in subterranean applications, e.g., in a well bore. As used herein, the terms “particulate” or “particulates” refer to a particle or particles that may have a physical shape of platelets, shavings, fibers, flakes, ribbons, rods, strips, spheroids, toroids, pellets, tablets, or any other suitable shape. The term “irreversible” as used herein means that the degradable material should degrade in situ (e.g., within a well bore) but should not recrystallize or reconsolidate in situ after degradation (e.g., in a well bore). The terms “degradation” or “degradable” refer to both the two relatively extreme cases of hydrolytic degradation that the degradable material may undergo, e.g., heterogeneous (or bulk erosion) and homogeneous (or surface erosion), and any stage of degradation in between these two. This degradation can be a result of, inter alia, a chemical or thermal reaction, or a reaction induced by radiation. The terms “polymer” or “polymers” as used herein do not imply any particular degree of polymerization; for instance, oligomers are encompassed within this definition.
The degradability of a degradable polymer often depends, at least in part, on its molecular structure. For instance, the presence of hydrolyzable and/or oxidizable linkages in the molecular structure often yields a material that will degrade as described herein. The rates at which such polymers degrade may be dependent on, among other things, the type(s) of repetitive units, composition, sequence, length, molecular geometry, molecular weight, morphology (e.g., crystallinity, size of spherulites, and orientation), hydrophilicity, hydrophobicity, surface area, and additives. Also, the environment to which the polymer is subjected may affect how it degrades, e.g., temperature, presence of moisture, oxygen, microorganisms, enzymes, pH, and the like.
The physical properties of degradable polymers may depend on several factors such as the composition of the repeat units, flexibility of the chain, presence of polar groups, molecular mass, degree of branching, crystallinity, orientation, etc. For example, short chain branches may reduce the degree of crystallinity of polymers while long chain branches lower the melt viscosity and impart, inter alia, extensional viscosity with tension-stiffening behavior. The degradability of a polymer can be further tailored by blending and copolymerizing it with another polymer, or by changing the macromolecular architecture (e.g., hyper-branched polymers, star-shaped, or dendrimers, etc.). The properties of any such suitable degradable polymers (e.g., hydrophobicity, hydrophilicity, rate of degradation, etc.) can be tailored by introducing select functional groups along the polymer chains. For example, poly(phenyllactide) will degrade at about one fifth of the rate of racemic poly(lactide) at a pH of 7.4 at 55° C.
Oftentimes degradable materials are commercially available in pellet form. However, for use in certain subterranean operations (e.g., as acid precursors, fluid loss control particles, diverting agents, filter cake components, drilling fluid additives, cement additives, etc.), it may be desirable to alter the average particle size of the degradable materials, among other purposes, to facilitate the dispersion of the materials in a slurry, and/or to control the reactivity and/or rate of reactivity of the degradable materials.
Thus certain processes may be desired to generate degradable particulates that can be transported to a job site and used in subterranean treatments. Common manufacturing processes that may produce such particulates include cryogenic grinding, which is an expensive process that involves grinding a degradable polymer, such as poly(lactic acid), at cryogenic temperatures to form particulates and powders having a desired shape and size. Oftentimes, these grinding processes are inefficient, requiring large volumes of liquid nitrogen and multiple passes through equipment, which usually results in yield losses. Moreover, cryogenic grinding methods generally are not useful for making degradable particulates that are smaller than about 150 microns in diameter. Also, mechanical classification (e.g., mechanical classification to separate particulates of differing sizes to obtain a specific size distribution) often is required to obtain narrow particle size distributions.
Another method that may be used to make degradable particulates off-site is spray drying. Spray drying processes usually involve dissolution of a degradable polymer sample in a volatile solvent (which can be an environmental problem itself), and spraying the solution into a stream of hot gas to make degradable particulates. However, mechanical classification and spray drying processes generally need to be carried out in a specially-designed factory setting, and the large scale production of degradable particulates of the desired sizes using these processes may not be practicable. Another method of producing degradable particulates is an extrusion method; however, extrusion methods generally are not useful for making degradable particulates that are smaller than about 500 microns in diameter. Moreover, some processes known in the art for generating degradable particulates utilize certain types of surfactants (e.g., sodium dodecyl sulfate) that may be effective in small-scale production methods, but may be less practicable for producing degradable polymer particles on a larger scale.